Automobile use is measured by tracking per capita motor vehicle ownership (including trucks) and estimates of C02 emissions from gasoline sold locally. CO2 emissions, one of the major drivers of global climate change, serve as a proxy for air quality impact of auto use. Emissions are estimated from county gasoline sales, some of which leaves the county.

Another issue with this indicator is that diesel fuel sales are estimates, while gasoline sales are calculated.  Also, the basis for calculating gasoline sales has changed during the years tracked. A more straightforward measure of the impact of transportation would be vehicle miles traveled, but this data is not available.

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